Fluid and osmolyte recovery in the common pond snail Lymnaea stagnalis following full-body withdrawal

The common pond snail Lymnaea stagnalis sacrifices 40-60% of its extracellular fluid (ECF) including solutes to the surrounding environment to fully retract into its shell. Our objectives were to determine recovery time following such ECF loss and characterize mechanisms involved in recovering Na+, the primary cationic osmolyte in this snail. Pallial fluid was initially collected post-stimulation and again after a second stimulation on sub-groups of those snails 2 h to 10 weeks after initial sampling. Samples were analyzed for pH, osmotic pressure, and individual solute concentrations. Lost volume was recovered within 8 h with no significant changes in pH or total CO2. Significant decreases in osmotic pressure, [Na+] ( 43% loss), and [Cl-] recovered in 48 h as a result of enhanced uptake from the water. Copper and total extracellular proteins took 5 weeks to recover. Measurements of Na+ transport kinetics completed before and immediately after fluid loss revealed a near threefold increase in both affinity and capacity of the Na+ uptake system. Sodium uptake was independent of ambient Cl- and HCO3- in both control and fluid-depleted snails. Amiloride significantly reduced recovery-phase Na+ uptake rates but did not influence baseline Na+ flux. Recovery uptake was significantly reduced by amiloride, ethylisopropylamiloride, bafilomycin and ethoxzolamide indicating dependency upon Na+/H+ exchange, H+ pump activity and H+ from carbonic anhydrase-catalyzed CO2 hydration. Thus enhanced uptake during recovery is likely via electrogenic Na+/H+ exchange and/or possibly a cation channel.